Method for producing water-atomized prealloyed powder with high cold press formability

ABSTRACT

A method for producing a water-atomized prealloyed powder with high cold press formability, includes the following steps: (a) preparing a −400 mesh semi-finished prealloyed powder; (b) controlling the semi-finished prealloyed powder to have a moisture content of 1 wt % to 2 wt % and an oxygen content of 0.6 wt % to 0.8 wt %, and then drying in a vacuum drying oven at 100° C. for 90 minutes to 120 minutes, so that a preliminary bond is produced between powder particles; and (c) reducing, annealing, crushing, and sieving an initially bonded powder particle. The powder is changed from a spheroidal shape to more complex shapes such as “rice ear shape”, “grape shape”, and “satellite powder”, which greatly improves the cold press formability of the prealloyed powder; the method only performs simple surface modification of the powder without changing other properties, and has wide applicability.

FIELD OF INVENTION

The present invention belongs to the technical field of powder metallurgy, and in particular relates to a method for producing a water-atomized prealloyed powder with high cold press formability.

BACKGROUND

Prealloyed powders for powder metallurgy and diamond tools are expected to have more complex particle shapes for good formability and high green strength. However, at present, the most easily mass-produced high-pressure water atomization pulverizing technology on the market generally produces powders with a spheroidal shape, an undeveloped surface structure, and poor cold press formability. In recent years, the widespread use of fully automatic machinery equipment have put forward higher requirements for the cold press formability of prealloyed powders, and the quality of cold press forming restricts the development of powder manufacturers. The existing improvement methods are as follows:

1. The morphology of powders is improved by controlling the atomization process, including adjusting the surface tension, viscosity and superheat degree of a metal, optimizing the combination of nozzles during high-pressure atomization, adjusting a spray medium, adjusting a spray angle, and adjusting the diameter of a molten metal flow (the diameter of a discharge spout), adjusting the flying distance and cooling time of a metal droplet, etc. This method has an effect on an individual prealloyed powder, but the scope of use is quite limited.

2. The powder is post-treated to achieve the purpose of easy cold press forming; there are two main ideas at present.

2.1. One idea is that a forming agent and a lubricant are directly added during the use of a prealloyed powder, but the forming agent and the like mixed into the powder need to be completely removed during a post-sintering process of a cutter head and a part, otherwise a small amount of residue will have a fatal effect.

2.2. The other idea is that a copper-containing oxide or salt is mixed into the prealloyed powder, then the non-metal component is removed by pyrolysis, and finally some of the remaining copper is plated on the surface of the prealloyed powder, so that the surface of the original spheroidal prealloyed powder is changed, producing a more complex structure and a larger specific surface area; besides, the copper has low hardness and good ductility, which greatly contributes to the cold press formability of the prealloyed powder; however, since the plated powder and the original powder are simply combined instead of being alloyed, in a strict sense, the powder produced by such a method is a mixed powder of a prealloyed powder and a simple powder, and its properties are far less than those of purely alloyed powders.

SUMMARY

An objective of the present invention is to provide a method for producing a water-atomized prealloyed powder with high cold press formability.

To achieve the above objective, the present invention adopts the following technical solution.

A method for producing a water-atomized prealloyed powder with high cold press formability, including the following steps:

1) preparing a −400 mesh semi-finished prealloyed powder, where the semi-finished prealloyed powder is required to be −400 mesh fine powder, because only a fine-grained powder particle has strong surface activity and large surface energy, which are prone to interface contact for the formation of a polymer;

2) controlling the semi-finished prealloyed powder to have a moisture content of 1 wt % to 2 wt % and an oxygen content of 0.6 wt % to 0.8 wt %, and then drying in a vacuum drying oven at 100° C. for 90 min to 120 min, where during this process, a preliminary bond is produced between powder particles by a capillary force; in this step, the moisture content of the semi-finished prealloyed powder is controlled to be 1 wt % to 2 wt %, in order to ensure a short vacuum drying process and improve production efficiency; the oxygen content is required to be 0.6 wt % to 0.8 wt %, in order to ensure that a subsequent reduction process can easily remove surface oxygen by using hydrogen, and improve the surface morphology of the particle; if the oxygen content is too low, the surface of the particle is not easy to form a pit and a pore; if the oxygen content is too high, the subsequent reduction process cannot reduce the oxygen content to a certain range, failing to meet a requirement for use; and

-   -   3) reducing, annealing, crushing, and sieving an initially         bonded powder particle. In step 1), the semi-finished prealloyed         powder adopts a water atomization pulverizing process, and the         specific operation thereof is: using high-pressure water to         crush a metallic solution into a micro droplet in an atomizer,         and filtering after cooling, where, the temperature of the         metallic solution is 1450 to 1750° C., the diameter of a nozzle         is 4 to 5 mm, a water flow intersection angle is 40°, a water         pressure is 65 Mpa to 80 Mpa, and a water flow rate is 180 L/min         to 200 L/min.

In step 1), the metallic solution is any one or a combination of two or more of iron, copper, nickel, tin, zinc, cobalt, tungsten, molybdenum, vanadium, and chromium.

In step 1), the semi-finished prealloyed powder is one of iron copper, iron copper nickel, iron copper nickel tin, iron copper cobalt tin, and iron tungsten molybdenum vanadium chromium.

In step 3), the reduction temperature is 500 to 600° C., and the reduction time is 8 h to 10 h, ensuring that the powder particle is slowly and fully reduced under a low temperature condition, and hydrogen and oxygen are effectively combined to achieve the effect of deoxidation; besides, the removal of oxygen in the powder particle makes it easy to form the pit and the pore on the surface, thereby increasing the specific surface area; the reduction can be carried out by putting into a push boat type or steel belt type reduction furnace.

In step 3), the annealing operation is: annealing at a temperature of 800 to 1050° C. and a vacuum degree of 10⁻¹ Kpa for 5 to 6 h; after the vacuum high-temperature annealing step, the powder particle polymer produced in the previous steps can be transformed into a crystal interface with a slight atomic bond. The transformation is more likely to occur at a vacuum degree of 10⁻¹ Kpa or below. The choice of temperature is important. If the temperature is low, the particles will not be bonded to form an agglomerate or have a certain strength; if the temperature is too high, the powder particles will be sintered, and mechanical properties and physical properties will be changed, which is contrary to the original intention of powder treatment.

In step 3), a continuous impact crusher is used to crush; the crusher has a speed of 2000 to 3000 rpm; this speed ensures that a powder particle after the crushing is microscopically an agglomerate of several powder particles and macroscopically has a more complex surface and have excellent cold press formability, ensuring that the crushed powder is not a single particle, but an agglomerate of several particles, so that the agglomerate macroscopically has a complex surface and is “rice ear-shaped” or “grape-shaped”.

In step 3), a 100 to 300 mesh screen is used to sieve to remove a coarse particle, so as to ensure the cold press formability of the finished powder, because an excessively coarse particle will break bonding uniformity between particles during the cold press forming process.

Compared with the prior art, the present invention has the following positive effects.

The present invention discloses a method for producing a water-atomized prealloyed powder with high cold press formability, which has the characteristics of simple and easy process, low cost and easy scale production. The present invention first uses a physical reaction to bond produced spheroidal powder particles, and then controls the crushing condition to crush the bonded bulk powder into a granular shape, and this process changes the powder from a spheroidal shape to more complex shapes such as “rice ear shape”, “grape shape”, and “satellite powder”, which greatly improves the cold press formability of the prealloyed powder; the method of the present invention only performs simple surface modification of the powder without changing the other properties, and has wide applicability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a process flow of the present invention;

FIG. 2 is a comparative scanning electron microscope image of an iron-copper prealloyed powder before and after being treated by the present invention (Embodiment 1); and

FIG. 3 is a comparative scanning electron microscope image of an iron-based prealloyed powder before and after being treated by the present invention (Embodiment 2).

DETAILED DESCRIPTION

The present invention is described in more detail below with reference to the embodiments and accompanying drawings.

Embodiment 1

A method for producing a water-atomized prealloyed powder with high cold press formability, with a process flow shown in FIG. 1, including the following steps:

1) weigh 50 kg of raw material required for the prealloyed powder production, including 70 wt % of iron and 30 wt % of copper; prepare a semi-finished prealloyed powder by a water atomization pulverizing process, where the temperature of a metallic solution is 1550 to 1600° C., the diameter of a nozzle is 5 mm, a water flow intersection angle is 40°, a water pressure is 80 Mpa, and a water flow rate is 200 L/min; use high-pressure water to crush a steel liquid into a micro droplet in an atomizer, and filter after cooling to obtain a −400 mesh semi-finished iron-copper prealloyed powder. In other embodiments, the metallic solution is any one or a combination of two or more of iron, copper, nickel, tin, zinc, cobalt, tungsten, molybdenum, vanadium, and chromium; preferably, the prealloyed powder is one of iron copper, iron copper nickel, iron copper nickel tin, iron copper cobalt tin, and iron tungsten molybdenum vanadium chromium.

2) control the semi-finished prealloyed powder to have a moisture content of 2 wt % and an oxygen content of 0.8 wt %, and then dry in a vacuum drying oven at 100° C. for 120 min, where during this process, a preliminary bond is produced between powder particles by a capillary force; in other embodiments, the semi-finished prealloyed powder may be controlled to have a moisture content of 1% to 2% and an oxygen content of 0.6 to 0.8%, and then dried in a vacuum drying oven at 100° C. for 90 min to 120 min.

3) put an initially bonded powder particle into a reduction furnace, which is a push boat type reduction furnace, select a reduction temperature of 550° C., reduce the powder particle under a hydrogen atmosphere, and treat for 8 h to remove surface oxygen, making the surface of the spheroidal metal particle covered with a pit and a pore to increase the specific surface area; then put the powder in a high-temperature vacuum annealing furnace, and treat for 5 h at an annealing temperature of 800° C. and a vacuum degree of 10⁻¹ Kpa, so that the powder is fully polymerized into a bulk; and use a continuous impact crusher to crush a powder agglomerate into a powder at a speed of 2000 rpm, and sieve through a 300 mesh screen to obtain a finished powder. The prealloyed powder obtained by the present invention and a comparative image are shown in FIG. 2. It can be seen from FIG. 2 that after an original particle of the prealloyed powder is treated by the method of the present invention, agglomeration occurs, and a single particle with complicated morphology is macroscopically displayed, which is actually an agglomerate formed by bonding a plurality of particles, and has excellent cold press formability. In other embodiments, in step 3), a steel belt type reduction furnace may be selected to treat at a reduction temperature of 500 to 600° C. for 8 to 10 h; then, the annealing process is adjusted to treat at a temperature of 800 to 1050° C. and a vacuum degree of 10⁻¹ Kpa for 5 to 6 h to sufficiently polymerize the powder into a bulk.

Embodiment 2

A method for producing a water-atomized prealloyed powder with high cold press formability, with a process flow shown in FIG. 1, including the following steps:

1) weigh 50 kg of raw material required for the production of an iron-based prealloyed powder for powder metallurgy, including 82 wt % of iron, 13 wt % of chromium, 1 wt % of molybdenum, 1 wt % of tungsten, 1 wt % of vanadium, and 2 wt % of carbon; prepare a semi-finished prealloyed powder by a water atomization pulverizing process, where the temperature of a metallic solution is 1650 to 1700° C., the diameter of a nozzle is 4.5 mm, a water flow intersection angle is 40°, a water pressure is 65 Mpa, and a water flow rate is 180 L/min; use high-pressure water to crush a steel liquid into a micro droplet in an atomizer, and filter after cooling to obtain a −400 mesh semi-finished iron-based prealloyed powder.

2) control the semi-finished prealloyed powder to have a moisture content of 1 wt % and an oxygen content of 0.6 wt %, and then dry in a vacuum drying oven at 100° C. for 100 min, where during this process, a preliminary bond is produced between powder particles by a capillary force.

3) put an initially bonded powder particle into a reduction furnace, which is a push boat type reduction furnace, select a reduction temperature of 600° C., reduce the powder particle under a hydrogen atmosphere, and treat for 10 h to remove surface oxygen, making the surface of the spheroidal metal particle covered with a pit and a pore to increase the specific surface area; then put the powder in a high-temperature vacuum annealing furnace, and treat for 6 h at an annealing temperature of 1050° C. and a vacuum degree of 10⁻¹ Kpa, so that the powder is fully polymerized into a bulk; and use a continuous impact crusher to crush a powder agglomerate into a powder at a speed of 3000 rpm, and sieve through a 100 mesh screen to obtain a finished powder. The prealloyed powder obtained by the present invention and a comparative image are shown in FIG. 3. It can be seen from FIG. 3 that after an original particle of the prealloyed powder is treated by the method of the present invention, agglomeration occurs, and a single particle with complicated morphology is macroscopically displayed, which is actually an agglomerate formed by bonding a plurality of particles, and has excellent cold press formability. The above embodiments are preferred implementations of the present invention, but the implementations of the present invention are not limited to the above embodiments. Changes, retouches, replacements, combinations and simplifications made without departing from the spiritual essence and principle of the present invention should be equivalent substitution manners, and should all be included in the protection scope of the present invention. 

What is claimed is:
 1. A method for producing a water-atomized prealloyed powder with high cold press formability, comprising the following steps: (a) preparing a −400 mesh semi-finished prealloyed powder; (b) controlling the semi-finished prealloyed powder to have a moisture content of 1 wt % to 2 wt % and an oxygen content of 0.6 wt % to 0.8 wt %, and then drying in a vacuum drying oven at 100° C. for 90 minutes to 120 minutes, so that a preliminary bond is produced between powder particles; and (c) reducing, annealing, crushing, and sieving an initially bonded powder particle.
 2. The method for producing a water-atomized prealloyed powder with high cold press formability according to claim 1, wherein in step (a), the semi-finished prealloyed powder adopts a water atomization pulverizing process, and the specific operation thereof is: using high-pressure water to crush a metallic solution into a micro droplet in an atomizer, and filtering after cooling, wherein, the temperature of the metallic solution is 1450 to 1750° C., the diameter of a nozzle is 4 to 5 mm, a water flow intersection angle is 40°, a water pressure is 65 Mpa to 80 Mpa, and a water flow rate is 180 L/min to 200 L/min.
 3. The method for producing a water-atomized prealloyed powder with high cold press formability according to claim 2, wherein in step (a), the metallic solution is any one or a combination of two or more of iron, copper, nickel, tin, zinc, cobalt, tungsten, molybdenum, vanadium, and chromium.
 4. The method for producing a water-atomized prealloyed powder with high cold press formability according to claim 2, wherein in step (a), the semi-finished prealloyed powder is one of iron copper, iron copper nickel, iron copper nickel tin, iron copper cobalt tin, and iron tungsten molybdenum vanadium chromium.
 5. The method for producing a water-atomized prealloyed powder with high cold press formability according to claim 1, wherein in step (c), the reduction temperature is 500 to 600° C., and the reduction time is 8 to 10 hours.
 6. The method for producing a water-atomized prealloyed powder with high cold press formability according to claim 1, wherein in step (c), the annealing operation is: annealing at a temperature of 800 to 1050° C. and a vacuum degree of 10⁻¹ Kpa for 5 to 6 hours.
 7. The method for producing a water-atomized prealloyed powder with high cold press formability according to claim 1, wherein in step (c), a continuous impact crusher is used to crush, and the crusher has a speed of 2000 to 3000 rpm.
 8. The method for producing a water-atomized prealloyed powder with high cold press formability according to claim 1, wherein in step (c), a 100 to 300 mesh screen is used to sieve. 